Apparatus and methods are known for the detection of growth of micro-organisms using a sterile nutrient medium in which are placed two electrodes (anode and cathode). The growth of a micro-organism placed in the nutrient medium can be detected by measuring the changing potential between the electrodes. It has been observed that when growth of the micro-organisms reaches a particular stage the potential shows a marked change, e.g. a steep fall.
In our British Patent Specification No. 2142433 the applicants have described an apparatus by which the growth of micro-organisms in a plurality of samples can be monitored electrochemically, by monitoring the potential difference developed galvanically between two electrodes of different metals, e.g. gold and aluminium, in contact with the samples.
Further, in the applicants' British Patent Application Publication No. 2211615 there is described a method of detecting micro-organisms in a cell in which an electrical D.C. input is applied across the two electrodes while maintaining constant current or constant potential conditions in the cell, and monitoring either voltage or current respectively in the cell against time. While these and other similar systems have been used successfully for detection of micro-organisms, the underlying mechanism giving rise to monitorable electrical changes has not hitherto been fully understood.
It has been discovered by the present inventors that the underlying mechanism involves the reduction of free dissolved oxygen at the cathode in the transfer of electrical charge between the electrodes. Whilst for aerobic samples using gold and aluminium electrodes there is a measurable potential of the order of 400-500 mV in the absence of micro-organisms, falling to 200-300 mV with the growth of micro-organisms and consumption by them of the dissolved oxygen, for anaerobic cells the initial measurable potential is extremely low or even zero because of the absence of dissolved oxygen. With this condition it is on the face of it impossible to use the known electrochemical detection techniques for anaerobic organisms.
The present invention has been developed to enable the above-described electrochemical detection techniques to be used for anaerobic organisms.